Optical transmission may be used as a means for communication between separate integrated circuit chips (inter-chip connections) and within components on the same chip (intra-chip connections). Electronic-photonic devices, also known as optoelectronic devices, are a class of electronic devices that are capable of sourcing, controlling, and/or detecting light. Electronic-photonic devices include both electronic and photonic functions. In response to more demanding communication bandwidth, energy consumption, and performance standards for electronic devices such as semiconductor devices, photonic devices are increasingly being integrated with optical/electrical circuits to form a type of electronic-photonic device called an electronic-photonic integrated circuit.
For example, in the semiconductor industry, photonic devices have various applications including communication within a chip, between chips of a computer board, and between computer boards. In chip-to-chip communication via optical interconnects, each chip on the circuit board can be interfaced with a photonic-electronic transmitter-receiver circuit, with two chips operably connected via an optical waveguide. Likewise, optical waveguides may be used to connect components within a chip, such as between an integrated optical source and a photonic detector. Another benefit of electronic-photonic devices is that the elements that perform the pure optical functions, the pure electrical functions and the optoelectronic functions may be formed concurrently, on the same or different substrate, using existing manufacturing processes such as complementary metal oxide semiconductor (CMOS) semiconductor manufacturing processes.
FIG. 1 illustrates a block diagram of one example of a conventional electronic-photonic device 100. Electronic-photonic device 100 may be used to operably connect elements, such as integrated circuits, on a single chip or substrate, or devices on separate substrates.
Electronic-photonic device 100 includes a light source 120 configured to generate an optical beam. Light source 120 may be, for example, a coherent light source, such as a laser (such as a hybrid silicon laser or a gallium arsenide laser), a coherent light-emitting diode (LED), a superluminescent diode, or other appropriate light source known in the art. A coherent light source is a light source typically having a narrow wavelength band that is consistent and in-phase. Light source 120 may be configured to output an optical beam having a wavelength in a range of approximately 1,200 nm to 1,550 nm.
An optical waveguide 130 connects the optical beam of light source 120 to a modulator 140, such as an optical ring resonator with a PIN junction. Modulator 140 modulates the received light beam with received electrical data 145, and outputs the modulated optical data along another waveguide 150. Modulator 140 is also capable of passing the optical beam through without modulation, such as when the optical beam has already been modulated by another modulator 140 in a same electronic-photonic system.
Photonic detector 160 includes a semiconductor material 162 (such as germanium (Ge), silicon germanium (SiGe), indium gallium arsenide (InGaAs), indium phosphate (InP) or other appropriate materials) that is configured to receive and collect the modulated optical beam. The electrical response is transmitted to one or more electrodes 164 that generate an electrical response upon receiving the energy of the wavelengths of the modulated optical data, and provide an external electrical connection for the received optical data.
FIGS. 2A and 2B show cross-sectional views of two examples of optical waveguides 150a, 150b, respectively. Optical waveguides 150a, 150b both include a respective inner core 152a, 152b and outer cladding 154a, 154b. 
Optical waveguide 150a (FIG. 2A) is an elliptically-shaped optical waveguide. Optical waveguide 150a is typical of a waveguide that may be formed as an optical fiber, such as a single mode or multi-mode optical fiber or other element separate from the substrate or chip to which the other photonic devices (e.g., light source 120, photonic detector 160, etc.) are formed. Outer core 154a may be, for example, a silicon dioxide (SiO2) material. Inner core 152a may be, for example, a silicon (Si) material, such as SiO2 doped with impurities such as GeO2, and typically has very small dimensions compared to outer cladding 154a. For example, inner core 152a may have a radius of approximately 9 μm, while outer cladding 154a may have a radius of approximately 125 μm.
Optical waveguide 150b (FIG. 2B) is a rectangular-shaped waveguide. Optical waveguide 150b is typical of an integrated optical waveguide that may be formed on a semiconductor, such as a silicon substrate, a silicon-on-insulator (SOI) substrate, or a printed circuit board (PCB), using lithographic processing. For example, an integrated optical waveguide 150b formed on a SiO2 substrate that acts as the outer cladding 154b may have a rectangular inner core 152b formed of, for example, a silicon (Si) material. Inner core 152b may have a diameter of approximately 300 nm, while outer cladding 154b is part of the larger substrate upon which optical waveguide 150b is formed and may have a diameter of approximately 1 μm or potentially much larger.
Wave guiding of an optical beam through waveguide 150a, 150b occurs through internal reflection of electromagnetic waves of an optical beam at the interface between the higher refractive index inner core 152a, 152b and the lower refractive index outer cladding 154a, 154b. Inner core 152a, 152b is formed of a material with a greater refractive index than the index of the material forming the outer cladding 154a, 154b. The refractive index of inner core 152a, 152b may be only slightly higher (e.g., 1%) than the refractive index of outer cladding 154a, 154b, or may be significantly higher (referred to as a “high contrast waveguide”) in order to provide greater total internal refraction (TIR). For example, inner core 152a, 152b may be formed of a silicon (Si) material with a refractive index of approximately 3.5, while outer cladding 154a, 154b may be formed of a silicon dioxide (SiO2) material with a refractive index of approximately 1.5.
It should be understood that outer cladding 154a, 154b can be formed of any material having a lower refractive index than the index of the inner core 152a, 152b. For example, ambient air, having a refractive index of approximately 1.0, may be used as outer cladding for an optical waveguide 150 having a Si inner core, and thus the cladding need not necessarily use a separate material. It should also be understood that both optical waveguides 130, 150 (FIG. 1) may have similar or different characteristics to those described above in connection with FIGS. 2A and 2B.
FIGS. 3A and 3B illustrate two top-down views of optical connections between an optical waveguide 150 and a photonic detector 160a, 160b. FIG. 3A shows a photonic detector 160a with optical waveguide 150 butt-coupled to the photonic detector 160a. Butt-coupled connections for photonic detectors require minimal length for the interconnection. However, the different refractive indexes between optical waveguide 150 and the semiconductor material of photonic detector 160a can cause energy from the optical beam to be reflected back into the optical waveguide 150. For example, optical waveguide 150 may be composed of Si having a refractive index of approximately 1.5, while photonic detector 160a may be composed of, e.g., Ge having a refractive index of approximately 4.34. This reflection is known as “return loss,” and in addition to diminishing the strength of the optical signal that is received by photonic detector 160a, can interfere with operation of light source 120 (FIG. 1).
FIG. 3B shows a photonic detector 160b with the optical waveguide 150 evanescent-coupled to the photonic detector 160b, which is composed of photonic detector portions 160b1 and 160b2. Photonic detector portions 160b1, 160b2 surround optical waveguide 150, but are separated from optical waveguide 150 by distances d1, d2, respectively. In evanescent coupling, optical waveguide 150 is placed close to photonic detector portions 160b1, 160b2 so that an evanescent field (i.e., a near-field standing wave formed at the boundary between inner core 152b and outer cladding 154b of FIG. 5B) generated by the transmission of the optical beam in optical waveguide 150 reaches photonic detector portions 160b1, 160b2 before fully decaying. Distances d1, d2 must be small enough that the intensity of the evanescent field from optical waveguide 150 does not fully diminish before it is detected by photonic detector portions 160b1, 160b2. For example, distances d1, d2 may be approximately 10 μm or less. The evanescent field from optical waveguide 150 gives rise to propagating-wave modes on photonic detector portions 160b1, 160b2, thereby connecting (or coupling) the wave from optical waveguide 150 to photonic detector portions 160b1, 160b2.
Evanescent-coupled photonic detectors 160b have lower return loss than butt-coupled photonic detectors 160a (FIG. 3A), but typically require longer path-lengths (e.g., approximately 50 μm or more) than butt-coupled photonic detectors 160a. This increases the footprint required for photonic detector 160b and thus the overall size of the electronic-photonic device 100 (FIG. 1).
Accordingly, it is desirable to provide an optical connection between an optical waveguide and a photonic detector with low return loss yet a small path-length.